Modulation of lipid metabolism, immune parameters, and hepatic transferrin expression in juvenile turbot (Scophthalmus maximus L.) by increasing dietary linseed oil levels

There is an increasing trend to replace fish oil (FO) in fish dietswith vegetable oils (VO), driven by the short supply of FO derived from wild fish stocks. However, little is known about its influences on fish health and metabolism. In the present study, FO was increasingly replaced by linseed oil (LO) in juvenile turbot's diet, with substitution levels at 0, 33.3%, 66.7% and 100%. The 66.7% FO could be replaced by LO without affecting fish growth and feed efficiency. Turbot in three LO replacing groups (33.3%, 66.7% and 100% LO) showed decreased body protein content compared to 100% FO group, while 33.3% LO group showed the highest body lipid content. In 100% LO group, both fatty acid desaturase and sterol O-acyltransferase2 gene expression significantly increased, while carnitine palmitoyltransferase 1 alpha gene expression significantly decreased (P < 0.05) compared to 100% FO group. These all suggest the inhibition of fatty acid beta-oxidation and promotion of lipid biosynthesis when FO was completely replaced by LO. Gene expression of sterol-regulatory element binding protein 1, which works as a regulating factor, significantly decreased in low replacing level (33.3% LO group) while remained similar to SREBP1 expression levels of the 100% FO group. Serum metabolites such as TCHO and HDL in 33.3% LO group were significantly higher than the 100% LO group, while serum AST activities significantly increased with increasing LO level (P < 0.05). For the immune parameters, both serum lysozyme activity and total antioxidant capacity in 100% LO group were significantly lower than 100% FO group, while superoxide dismutase activity was not significantly affected by fish oil replacement. Compared to 100% FO group, hepatic transferrin gene expression increased in 33.3% LO group but decreased in 100% LO group, which might be the reason of slight anaemia in 100% LO group. Further studies are still needed to illustrate the inner mechanism of transferrin regulation. Statement of Relevance: In commercial aquaculture, fish oil replacement has been widely conducted but less is known about its effect on lipid metabolism and immunity. Our research found that fish oil could be replaced by linseed oil at 66.7% in juvenile turbot's diet without affecting fish growth performance and immunity. (C) 2016 Elsevier B.V. All rights reserved.